The present invention relates to a method for controlling a switching device, in particular for synchronizing actuations of the switching device to a reference electrical signal; the present invention also relates to a control system adapted to carry out such method.
As known, a switching device is a device conceived for connecting/disconnecting two parts of an electrical circuit into which it is installed.
In particular, the switching device comprises one or more electrical phases, each one having at least one couple of contacts which can be switched between a closed condition, where the contacts are coupled to each other, and an open condition, where the contacts are separated from each other.
A control system can be provided for controlling the operations of the switching device, in such a way to synchronize the switching of the contacts to a reference waveform of an electrical signal associated to the electrical circuit into which the switching device itself is installed.
As known, the control system comprises control means which are adapted to operate by using a sequence of time cycles. The time cycles are set with a predetermined time duration.
The control means are adapted to control the actuation of the couple of contacts by using the time cycles with the predetermined time duration. The aim of this control is switching the contacts at a corresponding predetermined electrical angle of the reference waveform.
This predetermined electrical angle can be suitably chosen to avoid, or at least reduce, the generation of electrical arcs, inrush currents and transient voltages during the operation of the switching device.
However, the control means are adapted to execute the above mentioned control while assuming nominal values of relevant electrical and/or mechanical parameters which are associated to the phase and which could condition the desired synchronization of the contact switchings with the reference waveform.
If the real value of such electrical and/or mechanical parameters does not correspond to the presumed nominal value, the control means would fail to keep the desired synchronization as better illustrated with reference to an exemplary known switching device.
An exemplary known switching device comprises, for each electrical phase, two couples of contacts which are operatively associated to at least one semiconductor device.
The two couples of contacts must be switched in sequence at predetermined electrical angles of the reference waveform, in such a way to correctly use the semiconductor device for the switching tasks.
The two couples of contacts are realized by a common movable contact and two corresponding fixed contacts spatially separated from each other.
The movable contact can be actuated between a full-open position, where it is separated from both the first and second fixed contacts, and a closed position where it is coupled to the first fixed contact. The second fixed contact is disposed between the first fixed contact and the movable contact in the full-open position, so as to be connected with the movable contact during a portion of its travel path between the first and second fixed contacts.
An example of such switching device is disclosed in patent application EP2523203, filed in the name of the same applicant of the subject application.
The control means are set to control the actuation of the movable contact using the time cycles with the predetermined time duration, in such a way that:                the coupling of the movable contact with the second fixed contact starts at a first predetermined point and the coupling of the movable to the first fixed contact occurs at a second, subsequent, predetermined point of the reference waveform;        the separation of the movable contact from the first fixed occurs at a third predetermined point and the separation of the movable contact from the second fixed contact occurs at a fourth, subsequent, predetermined of the reference waveform.        
However, the control means are set to execute the above control while assuming a frequency value of the reference waveform equal to the frequency nominal value of the electric circuit.
In particular, the control means are adapted to apply a delay time between a detection of a predetermined reference point of the waveform and a predetermined starting point of the actuation of the movable contact.
This delay time is set according to the nominal frequency value and, hence, if the real frequency value does not correspond to such nominal value, the starting of the actuation of the movable contact will occur too early or too late with respect to the predetermined starting point.
More periods of the reference waveform the time delay comprises, and more the starting of the actuation will be far from the predetermined starting point.
In addition to such undesired effect, the control means are set to control the actuation of the movable contact while assuming a first preset time interval between the first and second predetermined points, and a second preset time interval between the third and fourth predetermined points of the reference waveform.
These first and second preset time intervals are based on the nominal frequency value.
Hence, a value difference between the real and nominal frequencies means a stretching or a reduction of the real time interval between the first and second predetermined points with respect to the first preset time interval, and a stretching or a reduction of the real time interval between the third and fourth predetermined points with respect to the second preset time interval.
This results in a controlled coupling between the movable contact and first fixed contact occurring too early or too late with respect to the second predetermined point, and in a controlled separation of the movable contact from the second fixed contact occurring too early or too late with respect to the fourth predetermined point of the reference waveform.
Furthermore, the control means are set to execute the control of the movable contact while assuming a distance between the first and second fixed contacts having a value corresponding to a nominal value devised in the design of the switching device.
However, the real value of such distance can vary in each single realized switching device with respect to the nominal designed value, due for example to mechanical tolerances. Since the control means work presuming the nominal distance value, a value difference between the real and nominal distances results in:                a coupling of the movable contact with the first fixed contact occurring too early or too late with respect to the second predetermined point of the reference waveform; and        a separation of the movable contact from the second fixed contact occurring too early or too late with respect to the fourth predetermined point of the reference waveform.        
Hence, all the above exemplary undesired effects combine each other resulting in a missed synchronization between the controlled actuation of the movable contact and the reference waveform.